In semiconductor fabrication, in order to achieve small feature sizes, for example, on semiconductor wafers for only one layer plane, it is possible to use a plurality of masks during successive exposures. In this case, the structure pattern to be formed on the wafer including a multiplicity of structure elements is usually projected into a photosensitive layer on the semiconductor wafer by a first mask.
A particularly high resolution is achieved if masks with improved lithography techniques (litho-enhancement techniques) are used. These may be, in particular, various types of phase masks, such as, for instance, alternating phase masks, halftone phase masks, tritone phase masks, chromeless phase masks, etc. However, the aforementioned techniques may also be ones which improve the imaging properties during the projection of mask structures, for instance, OPC (Optical Proximity Correction). The use of conventional COG (Chrome on Glass) masks is also provided, of course.
A second mask, also called trimming mask, is used in order to postprocess, i.e., to “trim”, the structures exposed in the photosensitive resist on the wafer through the first mask. In this case, the projection or exposure is effected partly into those regions in the photosensitive resist which have already been exposed through the first mask, but partly also into resist regions shaded by the first mask.
One advantage arises from the fact that it is possible to eliminate the side effects in the resist, which arise during the projection through phase masks. For example, the second trimming mask can be used to postexpose the shaded portions in the resist which arise at phase jumps of chromeless phase masks, insofar as the shaded portions are undesirable at specific positions on the wafer. This is because chromeless phase masks have the disadvantage that the regions provided with a phase swing on the mask have an edge region with a phase jump which represents a closed line. As a consequence, it is not possible to form arbitrary structures, in particular, lines ending in isolated fashion, arbitrarily in the resist. Through a postexposure, these closed lines on the wafer can be separated again through suitably positioned structure patterns on the trimming mask.
Alternating phase masks can be used to form particularly densely packed parallel lines in a layer on a wafer. In this case, on the mask, transparent regions with a first and a second phase swing are formed alternately in the interspaces between light-shading lines. The values for the respective phase swing typically differ by 180°, but are not restricted to this exact value. Where the light-shading lines end on the mask, the problem arises that the two transparent regions having different phase swings meet one another, so that the phase jump brought about in this region leads to an undesirable shading of the resist on the wafer during an exposure. Trimming masks are used for eliminating the problem.
The second masks of such a set, which are used as trimming masks, make it possible to use a particularly high-resolution mask type as first mask without themselves being subjected to these requirements. In particular, in a manner that saves costs, trimming masks used may be conventional masks which, under certain circumstances, are even exposed using mask writers of an older type. In the abovementioned cases, their task is rather to postexpose structures that are undesirable over a large area in a resist, the requirements made of the resolution being relatively low.
One disadvantage arises, on the other hand, from the fact that during a desired trimming, for example, of long lines formed by alternating phase masks on a wafer, the regions in the resist which are exposed through the first mask have an edge at which the radiation dose does not fall abruptly. This means that resist elements also outside the regions that are actually to be exposed have received an albeit low radiation dose. During a postexposure, through the trimming mask, the received radiation dose thereof is increased at a given position in the resist. Therefore, the exposed regions expand undesirably in the resist during a trimming mask exposure. Given extremely dense packing of closely situated lines on the first mask, it is then no longer possible to separate precisely one line from a multiplicity of closely adjacent lines. Rather, only double lines can be jointly separated.